Potential biomarker proteins for aspiration pneumonia detected by shotgun proteomics using buccal mucosa samples: A cross-sectional study

Abstract Background: Aspiration pneumonia (AP), which is a major cause of death in the elderly, does present with typical symptoms in the early stages of onset, thus it is difficult to detect and treat at an early stage. In this study, we identified biomarkers that are useful for the detection of AP and focused on salivary proteins, which may be collected non-invasively. This is because expectorating saliva is often difficult for elderly people, so we collected salivary proteins from the buccal mucosa.Methods: We collected samples from the buccal mucosa of six patients with AP and six control patients (no AP) in an acute-care hospital. Following protein precipitation using trichloroacetic acid and washing with acetone, the samples were analyzed by liquid chromatography and tandem mass spectrometry (LC-MS/MS). We also determined the number of cytokines and chemokines in non-precipitated samples from buccal mucosa.Results: Comparative quantitative analysis of LC-MS/MS spectra revealed 179 and 181 proteins that were significantly higher and lower in abundance, respectively, in the AP group compared with the control group. We identified eight proteins (chloride intracellular channel protein 3, b-defensin 4A, protein S100-A7A, Purkinje cell protein 4-like protein 1, glutamine synthetase, protein S100-A7, cytosolic 5'-nucleotidase 3A, pyridoxal kinase) that were detected in all of the AP samples with high coverage, indicating that these proteins are promising AP biomarker candidates. In addition, the abundance of C-reactive protein (CRP) in oral samples was highly correlated with serum CRP levels, suggesting that oral CRP levels may be used as a surrogate to predict serum CRP in AP patients. A multiplex cytokine/chemokine assay revealed that MCP-1 tended to be low, indicating unresponsiveness of MCP-1 and its downstream immune pathways in AP.Conclusion: Our findings suggest that oral salivary proteins, which are obtained non-invasively, can be utilized for the detection of AP.